BALTIMORE (WJZ) — Mapping what none of us can see might just help spot a future terror attack with a radioactive weapon.

Alex DeMetrick reports it’s an operation going on right now in Baltimore. All you have to do is look up.

It flies lower than most helicopters–the better to sniff out radiation. It made a number of passes over the terminals at the Port of Baltimore–just the kind of entry point experts worry a nuclear device or dirty bomb could come in.

Sky Eye Chopper 13 chased the specially equipped chopper.

“They’re probably flying a grid pattern. They’ve got to cover a lot of territory in a very short period of time,” said Captain Jeff Long.

But these flights aren’t chasing weapons–at least not yet. The National Nuclear Security Administration is mapping normal background radiation over Baltimore and other U.S. cities.

Background radiation comes from a lot of different sources.

Grading and construction use survey gear with radioactive elements. Radiation is also emitted in broadcast transmissions. And there isn’t a hospital that doesn’t have X-ray equipment and other radiological treatments and tests.

Mapping radiation levels now means a future radioactive threat can be located with another flyover, spotting a source that doesn’t belong.

“I think it’s good to be proactive. I think it makes me feel a little bit safer, although I hope certainly that nothing bad happens,” said Melissa Rocks, Baltimore.

When the survey is finished, if a nuclear threat is someday suspected, it could reduce a search for a weapon from days to hours.

Similar background radiation surveys are also being conducted in D.C. and some West Coast cities.